Fine particles comprising a polymer are utilized in various fields by making use of their characteristic features. The characteristic features include: particle diameter, mechanical strength, particle size distribution, shape, aggregation degree and the like, and fine particles are used by optimizing these characteristic features according to usage.
As the particle diameter of fine particles become smaller, its specific surface area is increased and greatly affects the characteristic features of the fine particle. Above all, fine particles having a particle diameter of 1,000 nm or less supposedly exhibit characteristic features utterly different from those of a fine particle of more than 1,000 nm, and attempts to microparticulate fine particles of various materials are being made.
The mechanical strength of fine particles affect their durability and are governed by polymerization degree, molecular weight, structure and the like of the constituent polymer. The mechanical strength is preferably as high as possible or has an optimal value for some uses. However, fine particles having low mechanical strength are greatly limited in their utilization and therefore, fine particles are generally required to have a certain level of mechanical strength.
The particle size distribution of fine particles can be regarded as variation in the characteristic features of the fine particles, because the characteristic features of the fine particles are greatly affected by the particle diameter. Therefore, a fine particle with higher uniformity of the particle diameter is desired except for some cases.
Fine particles are prepared in various shapes according to usage and may take various shapes such as spherical, flat, porous and indefinite, and fine particles having a shape appropriate for the purpose are desired.
Also, aggregation of fine particles with each other brings about a great change in the particle diameter and shape. The aggregation includes reversibly redispersible light aggregation and irreversible strong aggregation. However, generally, fine particles causing less aggregation are desired.
Although the combination of these characteristic features and its specific uses are of endless variety, examples thereof include a slipperiness-imparting agent, a toner, a delustering agent for paints, an additive for light diffusion, an antiblocking material for packaging mediums, an insulating filler, a crystal nucleating agent, a filler for chromatography, an abrasive, and other various additives. Furthermore, uses such as carrier for immunodiagnostic reagents, spacer for liquid crystal displays, standard particle for calibration of analyzers, standard particle for tests of porous film, or the like are also increasing recently.
In particular, particles used as a carrier for immunodiagnostic reagents, a spacer for liquid crystal displays, a standard particle for calibration of analyzers, a standard particle for tests of porous film, or the like are required to have a small particle diameter, sufficiently high mechanical strength, uniform particle size, high sphericity, and less aggregation of particles with each other. Fine particles having such characteristic features are known as monodisperse fine particles and are produced by a method such as emulsion polymerization, dispersion polymerization, seed polymerization and suspension polymerization. As for the material, polystyrene particles are being widely used.
However, the polystyrene is hydrophobic to exhibit bad dispersion stability in water and has a problem such as a change in particle diameter due to aggregation, or precipitation. Accordingly, in the case of liquid dispersion in water, addition of a dispersion stabilizer such as a surfactant, or a surface treatment is necessary to enhance the hydrophilicity. Furthermore, polystyrene is a material having very high solubility in an organic solvent and having a very low melting point and therefore, is disadvantageous in that, for example, it dissolves or swells in various organic solvents or cannot be used in an environment where heat is generated.
More specifically, the following problems are pointed out.
(1) In the case of use as a carrier for immunodiagnostic reagents, nonspecific adsorption is caused due to the presence of a surfactant and there arises a measurement error.
(2) In the case of use as a carrier for immunodiagnostic reagents, the fine particle adhered to the measuring cell is not easily flowed by water washing due to hydrophobicity and there arises a measurement error resulting from white turbidity in the measuring cell.
(3) In the case of use as standard particles for tests of porous film, fine particles adhered to the porous film do not easily flow when washed by water, due to hydrophobicity and there arises a measurement error resulting from change in the particle blocking rate caused by adsorption instead of the intended filtration.
(4) In the case of use as a standard particle for tests of porous film, the kind of the organic solvent which can be used as the liquid for filtration is limited because the fine particles dissolve or swell in many organic solvents.
(5) In the case of use as a material added to a molding of another material, the dispersion medium is limited because the fine particles dissolve or swell in many organic solvents.
On the other hand, cellulose has various characteristic features not found in the synthetic polymer such as polystyrene. Specific examples of the characteristic features include: (1) chemically relatively stable and difficult to dissolve, (2) heat resistant and not dissolvable even at a high temperature, (3) an amphipathic polymer having both hydrophilicity and lipophilicity, (4) natural product-derived and regarded as harmless to humans, (5) shapability and formability, (6) cause less interaction with a substance such as protein and cause no adsorption, (7) have many hydroxyl groups and allow easy chemical modification, (8) easily burn and generate no harmful substances, and (9) a biodegradable polymer and regarded as environmentally friendly.
Cellulose fine particles are applied to various uses by making use of the characteristic features of (1) to (9) above. Although the specific uses are of an endless variety, examples thereof include applications in many fields, such as packing material for various fractionation columns, enzyme support, microorganism culture carrier, cell culture carrier, filter medium, adsorbent, medicament excipient, medicament disintegrant, medicament extender, particle enlargement substrate, food thickener, thixotropy-imparting agent, dispersion stabilizer, plastic extender, filler, cosmetic foundation base, exterior paint modifier, coating agent, molding agent for catalyst production by firing method, fiber wall material, and compounding ingredient for pressure-sensitive copying paper. Also, it is known that when formed into a liquid dispersion, cellulose fine particles uniquely act with the dispersion medium and exert a peculiar effect on the behavior of the liquid dispersion. Furthermore, a cellulose derivative obtained by the chemical reaction of a hydroxyl group of cellulose is also applied similarly to various uses.
Cellulose fine particles having various characteristic features have been heretofore used according to the usage described above, and cellulose fine particles include, for example, cellulose fine particles obtained through physical pulverization or chemical pulverization and cellulose fine particles obtained through dissolution, formation of cellulose droplets, and coagulation-regeneration.
Examples of the former cellulose fine particles include those described in Patent documents 1, 2 and 3. However, the methods disclosed in these patent publications describe pulverization of randomly breaking down a polymer having a large structural unit, and the obtained cellulose is in most cases a bar-like or fibrous particle having a large L/D (D: particle diameter, L: length of the particle), which cannot be said to be a fine particle. The shape thereof is of course not uniform. In some reports, a fine particle having a small particle diameter to a certain extent is obtained, but reduction in the average polymerization degree of cellulose is involved for making small the particle diameter. In other words, in these cellulose fine particles, the small particle diameter and the high average polymerization degree are in an inversely proportional relationship. Furthermore, in general hydrolysis, the microparticulation has a limit derived from the level-off polymerization degree of cellulose and it is very difficult to obtain cellulose fine particles having a particle diameter of 1,000 nm or less. In the method of Patent Document 3, spherical cellulose fine particles having a particle diameter of 20 to 100 nm can be successfully obtained by hydrolyzing regenerated cellulose. However, considering the hydrolysis conditions described in the Examples, the average polymerization degree of the obtained cellulose particles is apparently decreased to about 50 which is the level-off polymerization degree of regenerated cellulose.
Examples of the latter cellulose fine particles include those described in Patent Documents 4 and 5. In these patent publications, cellulose fine particles having high sphericity have been reported. This method does not require to decrease the average polymerization degree of cellulose, and cellulose fine particles having a higher average polymerization degree than that obtained through hydrolysis is expected. However, mechanical force such as stirring or shearing is employed for forming fine droplets from a cellulose solution prepared by dissolving cellulose, and it is very difficult to obtain cellulose fine particles having a particle diameter of 1,000 nm or less. Even if a fine droplet to a certain extent can be formed by using a shearing apparatus such as ultrahigh pressure homogenizer, since cellulose needs to be dissolved in a solvent, the cellulose concentration in the fine droplet has an upper limit, and the cellulose fine particles obtained therefrom have a low apparent density and suffer from problems with strength, shape and the like. Furthermore, the fine particles obtained by such a method have a possibility that, for example, the size of the particle diameter is non-uniform or a surfactant, inorganic salt component or the like added at the time of forming fine droplets may remain.
Regarding the method for solving these problems, a method using the microphase separation described in Patent Document 6 is known. In this method, a particulate cellulose thick phase is prepared by dissolving cellulose in a good solvent and bringing about a microphase separation, and cellulose fine particles are obtained by performing coagulation-regeneration. The microphase separation is a method generally employed as a production method of a porous film formed by connected particles, where primary particles produced by a phase separation grow together into a larger secondary particle and secondary particles are connected with each other to form a porous film. In Patent Document 6, this principle is applied to the production of fine particles, but the obtained fine particles are secondary particles or a mixture of a primary particle and a secondary particle. Accordingly, the particle diameter of the fine particles is not sufficiently small and the size of the particle diameter is also not uniform.
Patent Document 6 provides cellulose fine particles having a number average particle diameter of 20 to 1,000 nm, obtained by decreasing the viscosity at 20° C. of a cellulose solution after dissolving cellulose, to thereby reduce the size of the secondary particle. Incidentally, in the patent publication above, the average particle diameter is expressed by the number average particle diameter and in the following, a value converted into an approximate volume average particle diameter predictable from the particle size distribution in the patent publication is set forth. In the patent publication above, reduction in the concentration and polymerization degree of cellulose dissolved, particularly, reduction in the polymerization degree, is indispensable for reducing the particle diameter of the cellulose fine particles. In other words, also in the patent publication above, the small particle diameter of the cellulose fine particle and the high average polymerization degree are in an inversely proportional relationship. Generally, a cellulose structure is considered to loose sufficient high strength when the average polymerization degree becomes 150 or less. In cellulose fine particles having an average polymerization degree of 150 or more described in the patent publication above, judging from the Examples, the average particle diameter exceeds 450 mm. When the average particle diameter is 450 nm, the minimum particle diameter and the maximum particle diameter in the particle size distribution are 40 nm and 1,000 nm, respectively. In other to decrease the average particle diameter, the average polymerization degree of cellulose needs to be further reduced, and the above-described cellulose fine particle is not a cellulose fine particle satisfying both a sufficiently small particle diameter and a sufficiently high average polymerization degree. Also, uniformity of the particle diameter is very low.
In this way, cellulose fine particles having a small particle diameter and a high average polymerization degree have not yet been provided. Of course, cellulose fine particles having all of the characteristic features, such as uniform particle size, high sphericity and less aggregation of particles with each other, have not yet been provided either. Cellulose fine particles having all of these characteristic features are expected to bring out a new function in the usage where cellulose fine particles have been heretofore used. Furthermore, cellulose fine particles are expected to become fine particles having, as monodisperse fine particles, hydrophilicity, organic solvent resistance and heat resistance, each in a high level that polystyrene fine particles cannot reach.
Patent Document 1: Japanese Examined Patent Publication No. 40-26274
Patent Document 2: Japanese Unexamined Patent Publication No. 3-163135
Patent Document 3: Japanese Unexamined Patent Publication No. 11-1719.01
Patent Document 4: Japanese Unexamined Patent Publication No. 61-241337
Patent Document 5: Japanese Unexamined Patent Publication No. 11-181147
Patent Document 6: Japanese Unexamined Patent Publication No. 61-211342